High humidity leads to loss of infectious influenza virus from simulated coughs

Nebulized influenza was coughed into the examination room and Bioaerosol samplers collected size-fractionated aerosols (<1 µM, 1–4 µM, and >4 µM aerodynamic diameters) adjacent to the breathing manikin’s mouth and also at other locations within the room. At constant temperature, the RH was varied from 7–73% and infectivity was assessed by the viral plaque assay.

Results

Total virus collected for 60 minutes retained 70.6–77.3% infectivity at relative humidity ≤23% but only 14.6–22.2% at relative humidity ≥43%. Analysis of the individual aerosol fractions showed a similar loss in infectivity among the fractions. Time interval analysis showed that most of the loss in infectivity within each aerosol fraction occurred 0–15 minutes after coughing. Thereafter, losses in infectivity continued up to 5 hours after coughing, however, the rate of decline at 45% relative humidity was not statistically different than that at 20% regardless of the aerosol fraction analyzed.

Conclusion

At low relative humidity, influenza retains maximal infectivity. Inactivation of the virus at higher relative humidity occurs rapidly after coughing. Although virus carried on aerosol particles <4 µM have the potential for remaining suspended in air currents longer and traveling further distances than those on larger particles, their rapid inactivation at high humidity tempers this concern. Maintaining indoor relative humidity >40% will significantly reduce the infectivity of aerosolized virus.

This recent study showed that humidity in the range of 40 -60%RH had a detrimental effect on the airborne influenza virus, rapidly rendering it harmless.

It reinforces the lower level of humidity in public and commercial premises ought to be maintained at above 40%RH to reduce airborne infection.

This is particularly true in environments like doctor's surgeries, hospitals and health care facilities where there are a greater number of infected individuals and others who are especially vulnerable.

The critical humidity level of 40 to 45%RH as lower limit has been reinforced over decades of research in countless studies.